I have a jump box that uses a 18v DC 500ma walwart .. I fired it by hooking it up to another jump box in order to charge it but didn't realize it needed 4amps vs 500ma so its fried .. But I am rebuilding the circuit and its really simply cause it has a step down transformer to a bridge recifer and then right to the jump box with a filter cap on the output.. But question is what limits the current cause I started using my power supply and the box will take 3 amps but the charging circuit gets pretty hot???

I think TS was asking what limits the current out of the wall-wart if it is connected to a higher current load than it was originally designed for?

A properly designed and UL approved Wall-Wart should stand an indefinite short-circuit on its output without starting a fire. Some have an internal fuse. Some have an internal self-resetting thermal breaker. Others use a saturating core in the transformer so that they are intrinsically current-limited.

That would be my guess as to "Jump Box". They come in all flavors and sizes. The worst case scenario is two different units where the charger for one is reverse polarity of the charger for the other. Guessing that could get ugly which brings us to Mike's post:

A properly designed and UL approved Wall-Wart should stand an indefinite short-circuit on its output without starting a fire. Some have an internal fuse. Some have an internal self-resetting thermal breaker. Others use a saturating core in the transformer so that they are intrinsically current-limited.

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You would think any of the above would hold true before bad things happen.

But question is what limits the current cause I started using my power supply and the box will take 3 amps but the charging circuit gets pretty hot???

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With a low battery the charger will deliver whatever it can as to a max current. As the battery becomes charged the current draw will slowly drop off. Most chargers like this are just very simple crude trickle chargers, connect them and let them charge overnight or longer. With a low battery the battery will draw whatever it can get from the charger. These are not well regulated charging devices.